The present system relates to a PCM-TASI signal transmission system in which the TASI (Time Assignment Speech Interpolation) system and the TASI techniques are applied to the time division multiplex PCM (PULSE CODE MODULATION) signals.
The TASI system is a transmission system aimed at reducing the number of transmission channels and has been contemplated by selectively connecting a large number of input trunks to the transmission channels depending on whether voice signals are present at each input trunk, and it has largely contributed to reduction of the per-channel transmission cost particularly for long-distance international telephone lines such as the Trans-Pacific submarine cable. With regard to the basic principle of the TASI system, a detailed description is given in Literature (1) below, and so, no further description will be given about it.
Literature-(1): E. F. O'Neill, "TASI" , Bell Laboratories Record, Vol. 37, No. 3, March 1959 p.p. 83-87 PA1 Literature-(2): C. Ota & K. Amano "A Digital Speech Interpolation System", Electronics & Communications in Japan, Vol. 56, No. 8, 1973, p.p. 26-34.
The principle of the TASI system can be applied not only to a frequency division analog transmission but also similarly to a time division multiplex PCM transmission, and it is extremely effective for radically reducing the number of transmission channels for the multiplex PCM signals. The latter application is called a PCM-TASI system, and R&D efforts in this field have been very active. With regard to the principle and construction of such a PCM-TASI system, a detailed description is made in literature-(2) below.
In a transmitter section of these TASI and PCM-TASI systems, four constituent elements are needed, that is, (1) a voice detector for detecting the presence of voice signals on the respective input trunks and for emitting requests for assignment of transmission channels, (2) means for deciding assignment between the input trunks and the transmission channels in accordance with the requests for assignment of transmisson channels from the voice detector, (3) means for informing the results of decision for assignment to a receiving section, and (4) means responsive to the results of assignment for controlling the switching between the input trunks and the transmission channels. The present invention relates, in particular, to the element (2) above, that is to a transmission channel assignment decision unit.
In a transmission channel assignment decision unit in the system disclosed in Literature-(2), monitoring of the transmission channels is carried out by means of an assigned channel number counter (CC), so that what can be monitored is limited solely to the number of channels and it is impossible to designate in connection with an assignment or unassignment of a particular transmission channel. However, under a practical operating condition, it can occur that a particular transmisson channel is desired to be monitored. It is highly desirable, for instance, in the case where the test signals are occasionally desired to be inserted forcibly into a particular transmission channel to check an error rate for the purpose of assuring reliability, or the case where the omission is desired of the particular transmission channel from practical use if said particular transmission channel is determined to be faulty as a result of the checking operation. As improvements in the above-referred transmission channel assignment decision unit, another transmission channel assignment decision unit has been known, in which in place of the above-referred assigned channel number counter there is provided a memory for storing possibilities of new assignment of the respective transmission channels so that a possibility of assignment can be arbitrarily designated for any transmission channel.
However, in these conventional transmission channel assignment decision units, since acceptance of assignment request signals from a voice detector is carried out in a sequence that is fixedly related to input trunk numbers, the frequency of rejection is different and non-uniform for every input trunk. Accordingly, provision is made such that the numbers of the input trunks whose assignment request signals cannot be accepted immediately are registered by means of a queuing buffer and they are processed with preference upon next assignment, whereby the probability of a particular input trunk being unable to be assigned with an idle transmission channel for a long period of time may be reduced. However, unless the capacity of the queuing buffer is as large as the memory capacity equivalent to the number of the input trunks, non-uniformity in processing among the input trunks would still remain. Even if the capacity of the queuing buffer is increased, the number of assignment requests that can be accepted at one time is limited, and therefore, merely increasing the buffer capacity for equalizing the input trunks in processing is extremely uneconomical. This is a first disadvantage in the prior art.
In addition, since the search for the transmission channels, that is, the read out of the memory for storing possibilities of new assignment of the respective transmission channels, is executed always in a fixed time sequence. The smaller the number of a transmission channel, the more frequently a new assignment is executed for that transmission channel. Still further, since provision is made such that once the assignment has been decided the same state of assignment is maintained so long as there remains a sufficient number of avilable transmission channels even if a voice signal should disappear from the input trunk for avoiding unnecessary disconnections, if the search for the transmission channels is always executed in a fixed sequence, then practically, transmission channels bearing smaller numbers are subjected to change in an assignment state immediately after the voice signal has disappeared. On the other hand, an input trunk that has been once assigned to a transmission channel bearing a larger number, would be scarcely subjected to release assignment. That is, assignment would become nonuniform and this is undesirable in view of the utilization of the transmission channels as well as transmission quality. This is a second disadvantage in the prior art.